The present invention relates to a compound powder or composite whose particles are comprised of ceramic or metallic whiskers, thinly embedded in a host or matrix material, the powder being provided ultimately for the manufacture of construction parts which are expected to undergo significant wear.
The term whiskers is going to be used throughout and includes extremely thin fibers within conventional meaning as well as very thin poly-crystalline fibers of relatively short dimensions which may have some deteriorated mechanical properties, as poly-crystals themselves have, but are, for some reason or another, more easily to be manufactured. Generally speaking, with increasing relative proportions of the whisker content in an embedding matrix, it is more and more difficult to obtain a homogenic distribution of the whiskers within the matrix, if the usual, known, powder metallurgical methods and steps of making such a product are used. In cases, actually, it may become impossible to provide a reasonably homogeneous distribution of the whiskers.
In the case of fiber re-inforced compound material, the inclusion and embedment of homogeneously distributed whiskers, produce a modified pattern in the formation of cracks or ruptures, such as crack deflection, crack splitting, whisker delamination or whisker breakage. Macorscopically, this amounts to a significant increase in ductility (k.sub.i factor). A concurrent improvement in strength depends to a considerable extent on the chosen composition of the material and here particularly the combination with a whisker embedding matrix. A limiting factor here is internal tension that is produced for one reason or another. Ductility has to be improved as well as the strength, and that depends basically, and again from a macroscopic point of view, on the volume fraction (ratio) of the whisker content. It can, therefore, be explicated through the following relation, which is, at least, approximately true: EQU .sigma..sub.c =x.sub..mu. +x.sub.w .sigma..sub.c.sbsb.w EQU K.sub.c =x.sub..mu. k.sub.c.sbsb..mu. +x.sub.w k.sub.c.sbsb.w
Contrary to poly-crystalline fibers which are quite widely used in the production of compound or composite material, and particularly of a ceramic and metallic nature, the whiskers have a significantly higher strength. Normally, it can be expected that these whiskers have a strength which is the eight to ten-fold strength value of poly-crystalline fibers. With this particular property, of course, has great significance for the compound material.
The effect of the previously mentioned mechanism for enhancing and increasing ductility and strength, depends to a considerable extent on the degree of homogeneity as far as the spatially or volume distribution of the whiskers in the embedding matrix is concerned. Areas with the significantly increased whisker content can be interpreted as macro defects and are, in fact, even at relatively low loads, prone to trigger component fracture.
Conventionally, whiskers are included in the making of powder by superimposing certain steps upon the powder metallurgical steps generally. First, the whiskers are cleaned in acetone, and then they are de-agglomerated with ultra-sound within n-Hexane alcohol or water. It has also been suggested to treat the whiskers, in addition, with a tumbling blender. Subsequently, ball grinding is used to mix whiskers with a powder matrix material. It is inevitable that under such circumstances whiskers brake. The fractured pieces will normally have rather sharp edges and will, in fact, subsequently, lead to significantly high local concentration and tension within the embedding matrix. On the other hand, a homogenous distribution of the whiskers becomes more and more difficult, the higher the proportion in whiskers, if the known techniques are used.